Since photographic materials are generally composed of a base having an electrically insulating property and photographic layers, static charges are often accumulated when producing photographic materials or using them in such a way that they are subjected to contact friction between surfaces of the same or different kinds of material or separation thereof. The accumulated static charges cause various troubles. The most serious trouble is that the light-sensitive emulsion layer is exposed to light due to the discharge of accumulated static charges prior to development which causes dot spots or resinous or feathery linear spots upon development of the photographic film. This phenomenon results in what is referred to as a static mark. Due to the creation of such marks the commercial value of photographic films is remarkably damaged or, sometimes, completely lost. For example, it is easily understood that static marks can result in a dangerous judgment when they appear on medical or industrial X-ray films. Since this phenomenon first becomes evident after carrying out development, it is a very troublesome problem. Further, the accumulated static charges cause secondary troubles, for example, dusts adhere to the surface of films or uniform application cannot be carried out.
Such static charges are often accumulated when producing photographic materials or using them, as described above. For example, they may be generated during production by contact friction between the photographic film and a roll or by separation of the base face and the emulsion face during winding or rewinding the photographic film. Further, they are generated in an automatic photographing apparatus by contact of the X-ray film with machine parts or with fluorescent sensitizing paper or separation therefrom. In addition, they are generated in contact with packing materials, etc. Generation of the static marks induced by accumulation of such static charges becomes very substantial as there is an increase in the sensitivity of the photographic materials and an increase in the rate of processing. Particularly, in recent years, static marks are more easily generated, because the photographic materials have high sensitivity and there are many opportunities for the material to be subjected to severe handling such as high speed application, high speed photographing or high speed automatic processing, etc.
In order to remove troubles due to static electricity, it is preferred to add antistatic agents to the photographic materials. However, it is impossible to use antistatic agents conventionally used in other fields because there are various restrictions which are characteristic to photographic materials. For example, antistatic agents which can be utilized in photographic materials must have excellent antistatic properties and not have a bad influence upon photographic properties of photographic materials such as sensitivity, fogging, granularity or sharpness. Further, they must not have bad influences upon film strength (namely, scratches are not easily formed by friction or scratching). It is also important that they not have bad influences upon antiadhesive properties (namely, the surface of the photographic material does not easily adhere to the surface of another photographic material or other materials), that they not promote fatigue of processing solutions for the photographic materials, or that they not reduce adhesive strength between the layers of the photographic materials. Accordingly, the application of antistatic agents to photographic materials is subjected to many restrictions.
One method of removing problems created by static electricity comprises increasing the electrical conductivity of the surface of photographic materials so as to disperse static charges in a short time prior to discharge of accumulated charges.
Thus, methods of increasing the electrial conductivity of the base of photographic materials or various kinds of surface coating layer thereof have been proposed. An attempt has been made at utilizing various hygroscopic substances and water-soluble inorganic salts, certain kinds of surface active agents and polymers. For example, the use of polymers described in U.S. Pat. Nos. 2,882,157, 2,972,535, 3,062,785, 3,262,807, 3,514,291, 3,615,531, 3,753,716 and 3,938,999, etc., surface active agents described in U.S. Pat. Nos. 2,982,651, 3,428,456, 3,457,076, 3,454,625, 3,552,972 and 3,655,387, etc., and metal oxides and colloidal silica described in U.S. Pat. Nos. 3,062,700, 3,245,833 and 3,525,621, etc., are known.
However, it is very difficult to apply these substances to photographic materials, because they are specifically adopted to one kind of film base or one type photographic composition. Accordingly, they produce good results in connection with certain specified film base and photographic emulsion or other photographic elements, but they are useless in preventing static charges with different film bases and photographic elements. Alternatively, these substances may have excellent antistatic properties but have a bad influences upon photographic properties such as sensitivity of photographic emulsions, fogging, granularity or sharpness. In addition, some of these substances have an excellent antistatic properties just after production but these properties deteriorate with the passage of time.
Nonionic surface active agents having one polyoxyethylene chain in a molecule are described in British Pat. No. 861,134 and German Pat. No. 1,422,809 and these agents have an excellent antistatic properties. However, when they are applied to the photographic materials, there are various problems such as: (1) they remarkably deteriorate sensitivity, (2) since their antistatic properties deteriorate with the passage of time, though they have a good antistatic property just after production, the antistatic properties of products become inferior when they are used, and (3) when they are applied to X-ray sensitive materials, dotted or mesh-like uneven density (which is called "screen contamination") is formed on the X-ray sensitive materials after development, because the X-ray sensitive materials contact with sensitizing paper (screen) in case of photographing. Accordingly, the value of the products is remarkably reduced and, sometimes, it is completely lost.
On the other hand, U.S. Pat. No. 3,850,641 has disclosed a method in which an ethylene oxide addition polymer of phenol-formaldehyde resin is applied as the antistatic agent for the photographic materials. This polymer is synthesized by carrying out a condensation polymerization of phenol derivatives and formaldehyde to form the so-called phenol-formaldehyde resin, and thereafter carrying out addition polymerization of ethylene oxide.
The phenol-formaldehyde resin synthesized as described above is inevitably contaminated by unreacted phenol derivatives. Contamination of the unreacted phenol derivatives becomes more remarkable when synthesizing a resin having a lower degree of polymerization. Further, the process for removing unreacting phenol derivatives in the resin is remarkably troublesome, because of the resin. Even if removal operation is repeated, it is very difficult to completely remove the unreacted phenol derivatives. Accordingly, it is essentially impossible to industrially produce phenol-formaldehyde resin which does not contain any unreacted phenol derivatives.
In an ethylene oxide addition polymer of the phenol-formaldehyde resin contaminated with unreacted phenol derivatives, it is impossible to avoid various problems analogous to those in nonionic surface active agents having one polyoxyethylene chain in the molecule described in British Pat. No. 861,134 and German Pat. No. 1,422,803. These problems are unavoidable because the polymer will include some molecules having one polyoxyethylene chain in the molecule originated in the unreacted phenol derivatives in addition to molecules having many polyoxyethylene chains in the molecule.
Further, it is very difficult to obtain phenol-formaldehyde resin having a definite composition, because not only the content of unreacted phenol derivatives but also the average degree of polymerization on the distribution of degree of polymerization varies according to a slight variation of conditions for synthesizing the resin. In addition, it is easily understood that, when the ethylene oxide addition polymer of phenol-formaldehyde resin is produced by addition polymerization of ethylene oxide, it is remarkably difficult to control the polymer so as to have a definite composition to form an antistatic layer having a definite quality.
Moreover, other phenol resins such as phenol-acetaldehyde resin or phenol-furfural resin, etc., have similar problems.